1. Field of the Invention
The present invention relates to a device for shielding a leakage of high frequency waves in a microwave oven, and more particularly to a high frequency wave shielding device for shielding high frequency waves outwardly leaked through a gap between a door and a front panel of a heating chamber of a microwave oven.
2. Description of the Prior Art
As well-known, microwave ovens are convenient kitchen appliances wherein high frequency waves generated from a magnetron are radiated on food placed in a heating chamber, so that the food itself generates heat by which the food is cooked. When a person is excessively exposed to high frequency waves, his body may suffer a fatal injury. It is, therefore, required to prevent the high frequency waves from being leaked out of the heating chamber.
Referring to FIG. 1, a general construction of a microwave oven is illustrated. As shown in FIG. 1, the microwave oven comprises a body 1 having a cavity 2 for receiving food to be cooked, a door 3 for opening and closing the cavity 2, and a window glass 4 attached to the door 3 and adapted to enable a user to see the interior of the cavity 2 therethrough.
For shielding a leakage of high frequency waves between a front panel 5 of the cavity 2 and the door 3 in the above-mentioned construction, a .lambda..multidot.g/4 choke channel 8 of a rectangular shape having a predetermined length L and a predetermined width W is defined by inner and outer metal panels 6 and 7, as shown in FIG. 2. The inner panel 6 extends upwardly up to an approximate middle portion of the choke channel 8 and is bent at its upper end toward the choke channel 8 so as to define an opening positioned over the inner panel 6. The opening is closed by a choke cover 10 attached at its inner surface with a ferrite rubber 9.
A capacitive seal 11 is formed between the front panel 5 of the cavity 2 and the inner panel 6 of the choke channel 8.
A transmission line path defined in a region at which the capacitive seal 11 is formed has a wave number km expressed by the following equation: ##EQU1## wherein, Ko represents 2.pi./.lambda. and b represents the space between the front panel 5 and the inner panel 2.
Where the value of b is very small, only the waves of the transverse electric magnetic wave mode (TEM mode) can be transmitted while the waves of the transverse electric wave mode (TE mode) and the waves of the transverse magnetic wave mode (TM mode) are shielded.
In other words, in case of the waves of TEM mode, the characteristic impedance of a parallel transmission line path is proportional to b.multidot..mu./.epsilon. (wherein, .mu. and .epsilon. represent a permeability and a permittivity, respectively). Accordingly, it is possible to reflect high frequency waves leaked out of the cavity 2 by greatly decreasing the value of b or increasing the value of .epsilon..
The lower horizontal portion 7a of the outer panel 7 constituting the choke channel 8 is a short surface exhibiting the impedance of zero. Assuming that the choke channel 8 is a transmission line path, the input impedance Zin at a position departed from a load ZL by a length l can be expressed by the following equation, as shown in FIG. 5: ##EQU2## wherein, Zo represents the characteristic impedance of the transmission line path and .beta. represents a phase constant determined by the construction of the transmission line path.
Assuming that ZL of the above equation is zero, the input impedance Zin corresponds to jZo.multidot.tan.beta.l. In this case, the input impedance Zin becomes the infinity (Zin=.infin.) when the length l is .lambda..multidot.g/4. When the length l is .lambda..multidot. g/2, the input impedance Zin becomes zero (Zin: 0).
As a result, where high frequency wave energy present in the interior of the cavity 2 is introduced in the interior of the choke channel 8 along a path B communicating with a point D during its leakage from a point A to a point B, the impedance at the point D can be determined by positioning the point D at a position departed a distance of .lambda..multidot. g/4 from the short surface exhibiting the impedance of zero. In this case, the choke channel 8 generates a resonance and thus reflects high frequency wave energy toward the cavity 2. As a result, it is possible to shield high frequency waves outwardly leaked.
In the above-mentioned conventional construction of the choke channel 8, however, the transmission of high frequency wave energy is achieved not only in the TEM mode, but also in the TE mode or the TM mode when the space between the inner panel 6 of the choke channel 8 and the front panel 5 of the cavity 2 is large. This requires the use of the .lambda..multidot.g/4 choke channel 8 and the expensive ferrite rubber 9 so as to prevent the leakage of high frequency waves. As a result, the manufacturing cost is increased. Furthermore, there is a drawback that the door 3 has an increased thickness because the choke channel 8 has a relatively large width and length.